TP6
This session invites contributions from all fields of planetary sciences that enlighten our understanding of the influence of late accretion and the physicochemical processes and conditions of planetary differentiation on the early evolution of rocky bodies, both in our own solar system, and in exoplanetary system. We especially encourage submissions from early career researchers.
Session assets
We have developed a comprehensive and self-consistent approach to simulate the late accretion and composition of terrestrial/rocky planes. Our approach begins with simulating the collisional growth of planetesimals and continues with resolving giant impacts using our state-of-the-art SPH-based model. We take into account all relevant physical processes including the dynamical friction due to the debris and planetesimal disks, migration of planetesimals and embryos, and the perturbation as well as possible migration of giant planets. As the most important step toward a fully comprehensive and realistic model, our approach incorporates SPH simulations into N-body integrations in real time allowing collisions to be simulated accurately as they occur. Results point to several important findings. For instance, in the context of our solar system, almost all simulations produced an Earth-analog. In the context of extrasolar planets, when giant planet migration was considered, rocky super-Earths were produced routinely. Simulations also show that the capture into resonance of migrating giant planets does not play a significant role on the formation of rocky planets. The most important finding of our simulations is that while giant planets may affect the inventory of planet-forming material, hence their compositions, they play no role in the mechanics of the formation of rocky planets and the transfer/transport of chemical compounds to them. Formation and delivery of chemical compounds are merely due to the mutual interactions of planetary embryos, a process that occurs even when no giant planet exists. We will present the results of our study and discuss their applications to the formation, composition, and early differentiation of Earth-like planets and rocky super-Earths.
Figure 1: Snapshots of SPH simulations of two different collisions. The impactor (smaller body, in blue) approaches the target from the left. The color coding represents different compositions, e.g., water on the impactor in blue and silicate material (basalt) in red. Iron core material (light gray). Top: Accretionary collision, graze-and-merge, where after a brief hit-and-run-like period the bodies fall back and merge. Bottom: Typical hit-and-run encounter with only relatively little volatile transfer. (Burger et al., 2020, A&A, 634, A76)
How to cite: Haghighipour, N., Burger, C., and Schäfer, C.: A Comprehensive and Self-consistent Approach to the Late Accretion and Composition of Rocky Planets, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-855, https://doi.org/10.5194/epsc2024-855, 2024.
Overview:
We investigate how both late accretion and long-term evolution of Venus are affected by early volatile exchanges (outgassing, loss, delivery), using a set of numerical models. In particular, we incorporate new scaling laws of large-impact erosion proposed by Kegerreis et al. (2020), and results of new atmosphere erosion simulations, using the open-source SPH code SWIFT. We assess the conditions and evolutionary pathways consistent with present-day observations of Venus and how to discriminate between late accretion scenarios.
Motivation:
Due to both the striking similarities and the obvious differences between Earth and Venus, understanding Venus might hold keys to how planets become – and cease to be – habitable. The question of the origin and persistence of water in the atmosphere/surface of Venus is directly linked to that of habitability. The divergence between Earth and Venus has been suggested to possibly occur during the first few hundred million years due to interaction between the interior of the planet, its atmosphere and escape mechanisms (e.g. Hamano et al., 2013).
General approach:
Late accretion impacts constitute both the tail end of the main delivery of material to the planet and, set after the magma ocean phase, the “initial conditions” for the planet’s long-term evolution. It could have important consequences for the distribution of elements, including volatiles. As no sample from Venus is available to constrain ancient history, we turn to modeling.
The major constraints on Venus’ volatile evolution come from the present-day state of its atmosphere and its bulk composition. We use different sets of late accretion scenarios and plausible volatile exchange mechanisms (sources and sinks). We compare the atmosphere composition of corresponding evolution models to the present-day state of Venus’ atmosphere. Particularly, we investigate the loss mechanisms (sinks of volatiles) since they impose an upper limit on the amount of volatiles that can be injected into the system while still reaching present-day composition. Required initial conditions are also discussed.
Models:
Three separate models are used together to investigate volatile exchanges during and following late accretion. Late accretion scenarios are obtained from N-body simulations. They include sequences of collisions with rocky planets leading to scenarios producing Solar System-like configurations and are calibrated using Earth’s late accretion mass based on the siderophile elements content of Earth’s mantle (Rubie et al., 2016). We used previously generated scenarios (see Gillmann et al., 2020) to test the consequences of different mass-size distributions on volatile evolution. We include new high-resolution simulations from Joiret et al. (2024), which include tracking 1600 carbonaceous asteroids and 10,000 comets.
Long-term evolution, interior evolution and atmosphere bulk composition are tracked using StagYY mantle dynamics models (see Gillmann et al., 2020). Important volatile-exchange mechanisms include volcanic outgassing, thermal and non-thermal atmosphere escape, and gas–surface chemical reactions through oxidation of fresh lava. Coupling between the interior and the atmosphere is obtained by tracking surface temperature evolution using a radiative convective grey atmosphere model. Impacts affect the atmosphere in three different ways: (i) collisions cause the mantle and surface to melt, releasing volatiles, (ii) impactors deliver volatiles to the atmosphere as they are vaporized depending on their composition, (iii) the impact process leads to atmospheric erosion through a variety of mechanisms.
Previous results were obtained using (i) a geometric approach, through the tangent plane model or (ii) using small-impactor results from the SOVA hydrocode (Shuvalov et al., 2014). Here we investigate the importance of constraining impact erosion of the atmosphere by comparing those previous results to more recent larger-impactor scaling laws obtained by Kegerreis et al. (2020). We further include ongoing simulations specifically developed for Venus’ atmosphere compositions and specific collisions defined by the late accretion impact scenarios described above.
Results:
The tiny amount of water in the present-day atmosphere of Venus limits water delivery from various mechanisms, even when considering water sinks throughout the history of the planet. The maximum amount of water that can be delivered, in turn, governs the estimated overall composition of late accretion impactors, imposing that the bulk of late accretion should be volatile poor.
Non-thermal loss mechanisms can account for the loss in the range from 4 mbar up to a few bar of oxygen, depending on assumptions, and over >4 Gyr. Trapping oxygen on the surface through oxidation of newly emplaced volcanic material through solid–gas reactions appears inefficient (for a total loss similar to non-thermal escape), while recent oxidation of impact ejecta is a comparatively even smaller sink.
On the first order, scaling laws extracted from simulations by Kegerreis et al. (2020) imply increased losses for high-energy collisions compared to previous estimates (Shuvalov et al., 2014), but use a different set of assumptions for the investigated atmospheres and consider larger bodies (figure 1). We investigate a series of possible parameterizations to reconcile those results.
Figure 1: Loss rates for single impacts calculated from the two sets of simulations described in the text.
Using lower atmosphere-erosion estimates leads to similar results regardless of the mass-size distribution of impactors, because the impact delivery of volatiles is the overall dominant effect. On the other hand, strong atmosphere erosion introduces divergences between late accretion scenarios depending on the distribution of impactor sizes, with smaller impactors having a net destructive effect while larger impactors contribute to increasing the atmosphere mass. This effect is not apparent in the water inventory but instead can be seen in the CO2 abundance evolution (figure 2). Therefore, we use CO2, N2 and Ar abundances to further discriminate between possible late accretion scenarios.
Figure 2: Evolution of CO2 abundance in the atmosphere from simulations of two scenarios for late accretion. Scenario B includes a few large impactors, scenario D contains a larger number of smaller bodies.
References:
Kegerreis, J. A., et al. (2020), Astrophys. J. Let. 901.2.
Hamano, K. et al. (2013), Nature 497, 607-610.
Rubie, D. C. et al. (2016), Science 353, 1141–1144.
Gillmann, C. et al. (2020), Nature Geoscience 13, 265–269
Joiret, S. et al., (2024), Icarus 414.
Shuvalov, V. et al. (2014), Planet. Space Sci. 98, 120-127.
How to cite: Gillmann, C., Golabek, G., and Kegerreis, J.: Consequences for the early evolution of Venus from new simulations of atmosphere erosion by impacts., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-357, https://doi.org/10.5194/epsc2024-357, 2024.
After the magma ocean phase of a rocky planetary body, global partial melt zones contribute significantly to the formation of a planet’s secondary crust. During the partial melting inside the upper mantle, trace elements and volatiles that are incompatible with the surrounding mantle rock are redistributed from the solid into the liquid material. If the melt is buoyant, it will rise up towards the surface, resulting in an enriched crust and depleted upper mantle.
The quantity of the redistributed elements can be determined by mineral/melt partition coefficients (D), which are a measure of the compatibility of an element inside a crystal lattice. Generally, partition coefficients are dependent on pressure, temperature, and composition. Since there is a lack of both, high-pressure experiments and models, older mantle evolution models usually take their values as either a constant value from the literature or a constant assumed model value. For example, as an assumed model value for water, mantle evolution models often choose a partition coefficient of D=0.01.
In this study, we applied a partition coefficient model that is applicable to higher upper mantle pressures (Schmidt and Noack 2021) to a thermo-chemical evolution model for rocky planets. We found that mineral/melt partition coefficients 1) inside a partial melt zone of a rocky planet and 2) between the melt zones of different planetary bodies can change up to one order of magnitude, sometimes reaching even two orders of magnitude (Figure 1, Schmidt et al. 2024). We investigated the influence of these finding on the redistribution of heat producing elements (HPE) and water and evaluated their effect on the thermal evolution and outgassing of a planet (Figure 2, Schmidt et al 2024).
Further studies on the inclusion of partition coefficients into interior evolution models for stagnant lid planets show that there is a linear correlation between partition coefficient and planet size. This, in turn, has an especially large impact on the redistribution of water and outgassing into the atmosphere of super-Earths (Schmidt and Noack, 2024).
Schmidt, J.M. and Noack, L. (2021): Clinopyroxene/Melt Partitioning: Models for Higher Upper Mantle Pressures Applied to Sodium and Potassium, SysMea, 13(3&4), 125-136.
Schmidt, J.M., Vulpius, S., Brachmann, C., Noack, L. (2024): Redistribution of trace elements from mantle to the crust in rocky solar system bodies, to be submitted.
Schmidt, J.M., Noack, L. (2024): Planet mass controls the mineral/melt partitioning of trace elements in the upper mantle of rocky planets, in preparation.
Figure 1: Range of mineral/melt partition coefficients for Cerium inside global partial melt zones of modeled solar system rocky bodies. All bodies are assumed to be in a stagnant lid tectonic regime, except ML Earth, which we model as a mobile lid regime planet (Schmidt et al. 2024, to be submitted).
Figure 2: Equivalent Global layer thickness variations for Moon, Mars, and Venus. The EGL describes a hypothetical scenario if all outgassed water condensed into an ocean. It is highly dependent on the amount of redistributed water towards the crust (Schmidt et al. 2024, to be submitted).
How to cite: Schmidt, J. M. and Noack, L.: Redistribution of trace elements from the mantle towards the crust in rocky planetary bodies and implications for the thermo-chemical evolution and outgassing, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-541, https://doi.org/10.5194/epsc2024-541, 2024.
Introduction: The chemical composition of magmatic iron meteorites provides fundamental insights into planetary accretion processes. They are distinguished based on their trace element compositions and could represent the cores of more than 50 parent bodies [1]. The primary difference between the different groups is the degree of volatile element depletions, which increases from class I to IV iron meteorites [2]. For example, Cu, Ge and Ag concentrations of magmatic iron meteorites deviate up to 4 log units between the different magmatic iron meteorite groups [2]. The volatile element loss could have occurred prior to (i.e. nebular) or during parent body accretion and differentiation, for example during exposure of a liquid core following a catastrophic impact [3]. Investigating the mechanisms of volatile loss from requires experimental constraints on their volatility, for example during evaporation from metallic melts [3]. We previously determined the volatility of Cu, Ge, Ag and S [4]. Here, we extend the latter work to include most other volatile elements (Zn, Ga, Se, Cd, In, Sn, Sb, Te, Pb, Bi), and experimentally determined their evaporation from metallic melts as a function of pressure (10-4 to 1 bar), temperature (1573−1773 K) and time (5−120 min) for two end-member compositions (Fe versus FeS).
Approach: Evaporation experiments were performed in a high-temperature (vacuum) furnace. Experimental starting compositions consisted of metallic Fe or FeS starting powders doped with the (trace) elements of interest. After the required run time experimental run products were quenched in water, mounted in epoxy resin, polished and prepared for subsequent analyses by electron microprobe and LA-ICP-MS at the University of Münster. The LA-ICP-MS analyses were calibrated using the NIST 610 glass and 56Fe as an internal standard. A spot size of preferably 130 μm was used for virtually all analyses. To obtain reference concentrations of the starting materials prior to degassing, starting materials were synthesized at high P-T conditions in a piston cylinder press. The measured concentrations in the experimental run products were then normalized relative to the elemental concentrations measured in the undegassed, high P-T synthesized metal and sulfide liquids. Note that this approach also rules out any potentially significant matrix effects on LA-ICP-MS derived concentrations of degassed samples: both materials are (near)-identical in major element compositions [4].
Results and discussion: Fig. 1 shows a typical experimental run product. Experiments performed at room pressure generally quenched to a single blob, whereas vacuum experiments occasionally dispersed into several smaller metallic blobs upon quenching. All experimental phases, including different blobs from the same experiment, were found to the un-zoned, suggesting that the evaporation of these elements from metallic melts is not diffusion-limited (Fig. 2).
Figure 1: Examples of experimental run products
Figure 2: Concentration profiles along rim-to-rim and/or vertical LA-ICP-MS spot transects in experimental samples
The volatility or evaporative loss factor of element i (fi) was defined using the following equation: concentration of element i in experimental sample (ppm) / concentration of element i in reference material (ppm) [4]. These factors were then normalized to Ni, a non-volatile element within the explored P-T conditions, by considering f (i/Ni), defined as f i / f Ni. Figure 3 shows an example of these values for Pb in FeS liquid.
Figure 3: Experimentally determined f (Pb/Ni) values for FeS liquid
The new experimental data confirmed previous hypothesis related to the importance of S in establishing elemental volatilities (e.g., ref. 4). Some samples were so rapidly degassed that only minimum evaporative loss factors could be calculated (e.g., for Cd). Using the new experimental data, parameterizations were obtained that can be used to predict the volatility of the elements of interest [4,5] in FeS or S-free alloys as a function of T and time at a vacuum of 10-4 bar and/or 1 bar. These parameterizations were applied to predict the evaporative volatile loss during different planetary differentiation scenarios. A comparison of the new results with traditionally applied volatility models based on condensation temperatures confirms that the latter models are only partly applicable to constrain evaporative loss and particularly such loss from metallic melts [4]. The new data and parameterizations will be discussed at the meeting in light of current models that describe volatile element depletions in magmatic iron meteorite parent bodies.
References
[1] Goldstein et al. (2009) Chemie Der Erde – Geochem [2] Scott & Wasson (1975) Rev Geophys [3] Kleine et al. (2018) LPSC #2083 [4] Steenstra et al. (2023) EPSL [5] Steenstra et al., under review.
How to cite: Steenstra, E. S., Renggli, C. J., Berndt, J., and Klemme, S.: The evaporation of volatile elements from metal melts: implications for volatile element depletions in metal-rich planetesimals, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-185, https://doi.org/10.5194/epsc2024-185, 2024.
Introduction
After the formation of the Moon its surface was likely covered by a global lunar magma ocean (LMO). It is typically assumed that this LMO crystallized via bottom-up fractional solidification, where crystallized material accumulated at the bottom of the LMO. This leads to a layered mantle structure, where the early cumulates are dominated by olivine, then pyroxenes, and after about 60-70% of LMO crystallization plagioclase forms, which has a lower density and floats to the surface to build the crust. At the end of the solidification, the heavy elements such as iron and titanium crystallize, as well as the KREEP components and the heat-producing elements. As the density of the solidified material increases towards the surface, the mantle is prone to overturn. During and following the overturn, mantle material melts where the temperature exceeds its solidus. These melts rise to the surface and form the secondary crust, i.e., Mg-suite and mare basalts.
In this study, we investigate the formation of the secondary crust components and use their amount and timing of formation as constraints in our simulations. The amount of Mg-suite rocks is estimated to be 6-30% (Prissel et al. 2023, Warren & Kallemeyn 1993) and the amount of mare basalts to be 1-2% (Broquet & Andrews-Hanna 2024). The timing and duration of the Mg-suite rocks dated so far is very concise with 4348±25 Ma ago (Prissel et al. 2023) compared to the longstanding activity of the mare basalts between 3.9 – 1.2 Ga (Hiesinger et al. 2011).
Previous studies
In thermochemical evolution models studying the secondary crust formation, a common assumption for the structure of the Moon after LMO solidification is a homogeneous, peridotitic mantle and an anorthositic crust, sometimes with a layer of ilmenite bearing cumulates (IBC) beneath the crust. (e.g., Ziethe et al. 2009; Laneuville et al. 2013, 2018; Yu et al. 2019). Initially, after crystallization of the LMO, the temperature is typically set to the solidus temperature. These studies show longstanding volcanic activity also as a consequence of an insulating crust but generally result in far too high amounts of secondary crust - as also confirmed in our work. Recently, Prissel et al. (2023) have studied the formation of Mg-suite and find that the rapid rise and melting of the olivine cumulates due to mantle overturn can explain their formation shortly after the crystallization of the LMO. Their model, however, does not consider mare basalt formation and assumes the initial temperature to be equal to the solidus temperature after LMO solidification - as the previous models described above.
Petrological Model
In our petrological model (Schwinger & Breuer 2022), we calculate a layered mantle structure assuming fractional solidification of a global magma ocean of 1350 km. As initial composition of the lunar magma ocean we consider the composition of O'Neill (1991) with 10 wt% FeO. The complex stratification has been approximated by five main layers (Fig 1c). For each of these layers the density, the thickness, and for mantle layers also the solidus and liquidus are calculated.
We investigate different approaches to get a realistic solidus, using MELTS and comparing the results to the phase diagrams (Fig. 2). Within the cpx-layer the density and the melting temperatures change substantially. Therefore, we additionally investigate a mantle structure, where the cpx layer is further divided in three sublayers (not shown in Fig. 1). We consider the change in density and solidus due to secondary melting for each mantle layer. Furthermore, the solidification model provides the crystallization temperature that is used as initial temperature profile for the geodynamic models.
Geodynamic Model
For the thermochemical evolution models, we use the mantle convection code GAIA (Hüttig et al., 2013). In addition to the layered initial lunar mantle structure, we consider also a homogeneous mantle structure with peridotitic composition for comparison. We employ an Arrhenius law to calculate the depth- and temperature-dependent viscosity, and we consider core cooling and radioactive decay. The material of the different layers mixes due to solid-state convection. We employ a particle-in-cell method (Plesa et al., 2013) to track the mixing, where the tracer particles carry information such as the composition, the density, the mantle depletion, and the melting temperatures of the material. We consider the influence of latent heat consumption during melting and the changes in residual materials due to secondary melting. The amount of melt is calculated using a linear interpolation between the solidus and the liquidus.
Results and Discussion
The results show that both the rapid formation of the Mg-suite rocks and the mare basalts, with ages as young as 1-2 Ga ago, are difficult to achieve with the current models. Neither a homogeneous mantle with peridotitic composition (too much crust is produced or too late in the evolution) nor a layered mantle due to fractional crystallization (olivine cumulates as a reservoir for the Mg-suite cannot be molten and crust formation ends too rapidly) can explain the observations. In the models with a layered mantle, the main reason is the difference between the crystallization temperature and the melting temperatures as a consequence of the fractional crystallization. Thus, our results do not support the findings of Prissel et al. (2023) who suggest that the formation of the Mg-suite is caused by the lunar global overturn of the fractionated LMO. In these models, however, the authors assume that the crystallization temperature is equal to the solidus temperature – which is not consistent to the petrological model (cf. Fig. 1).
To explain the observed volcanic history, we conclude that pure fractional crystallization of the LMO seems unlikely. We discuss possible ways to produce the Mg-suite and prolong the formation of mare basalts, including batch crystallization of the deep magma ocean, the onset of mantle convection already during solidification of the LMO, the presence of a primitive mantle layer beneath the crystallized magma ocean, but also the influence of water and melt in the retention.
Acknowledgements
I.B. and S.S. were supported by DFG SFB-TRR170, (subprojects C4 and A5).
How to cite: Bernt, I., Schwinger, S., Plesa, A.-C., and Breuer, D.: Moons crust evolution: combining petrological with geodynamical models, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1242, https://doi.org/10.5194/epsc2024-1242, 2024.
Introduction: One of the main characteristics of lunar rocks is their strong depletion in moderately volatile elements (MVE) compared to Earth [1], as well as a strong enrichment of heavy isotopes of the MVE, e.g. Cu, Zn, Rb, K, Ga [2-6]. These characteristics have been taken as major arguments in favor of catastrophic outgassing during the Moon forming impact [7], lunar magma ocean (LMO) evolution [8], and/or subsequent magmatic evolution [3]. Due to their ineffectiveness in producing a global MVE depletion and associated isotopic fractionation in the lunar mantle, the two last hypotheses have been questioned in the light of new cutting-edge ab initio calculations [9]. In addition, whereas some of the MVE, e.g. K, Rb, display relatively homogeneous mass-dependent isotopic composition in mare basalts (MB) there appears to be more isotopic heterogeneity for others, particularly Cu [2]. The exact reasons for this isotopic heterogeneity in MB are unknown and may lie in the siderophile/chalcophile behavior of Cu or local volatility-related processes. Here, we present new Cu isotope data for 30 MB and discuss whether their isotopic composition is related to processes during formation of their mantle sources, basalt formation or accompanying volatility-related processes.
Samples and methods: 300 to 500 ng of Cu have been purified from 100 mg low-Ti and high-Ti MB from Apollo 11, 12, 15, and 17 following a chemistry specifically designed for Ti-Fe-rich samples. Copper isotopic ratios were obtained by a NEOMA MC-ICP-MS at Freie Universität Berlin and are reported as delta values (i.e. δ65Cu in ‰) relative to the ERM®-AE633 standard, with uncertainties at 95% confidence intervals.
Results: Compared to low-Ti MB ([Cu]: 4.07 µg/g to 8.55 µg/g; δ65Cu: 0.22 ± 0.04 ‰ to 1.28 ± 0.06 ‰), high-Ti MB have a significantly lighter δ65Cu for a given [Cu] ([Cu]: 2.70 µg/g to 5.65 µg/g; δ65Cu: -1.42 ± 0.10 ‰ to 0.75 ± 0.01 ‰). In addition, most of the MB suites define a narrow range of [Cu]mean and δ65Cumean.
Discussion: Copper content correlates between and within some suites with tracers of magmatic evolution (MgO, FeO, TiO2, Cr/Sm). In contrast, δ65Cu does not correlate with MgO, TiO2, and Cr/Sm and its variation within and between mare basalt suites cannot be explained by silicate fractionation. This strongly suggests that [Cu] changes in different MB suites is a collateral effect of silicate fractionation which agrees with the incompatible nature of Cu, whereas δ65Cu variations in a few suites may reflect mixing of magmas from isotopically different reservoirs and/or local volatility-related processes. However, considering the latter possibility, the range of δ65Cu compositions of MB can only be reproduced if unrealistic initial [Cu] are used, with isotopic fractionation factors very close to unity. Hence, we rule out evaporation during volcanic eruptions as the main process controlling δ65Cu heterogeneity in MB.
Interestingly, MB with the heaviest δ65Cu and highest Mg# have also the lowest S/Cu, S/Se, S/Te, S/Ag and S/Pd ratios (Fig. 1). As S is less chalcophile and siderophile than Cu, Se, Te, Ag, and Pd, and Δ65Cumetal-sulfide > 0 [11], low S/Cu, S/Se, S/Te, S/Ag, S/Pd with heavy δ65Cu in MB should be characteristic of S-poor FeNi alloys while higher ratios generally match lighter δ65Cu expected for sulfides. Such systematics exist from low-Ti to high-Ti MB but not within most of the individual MB suites. Instead, most of the suites define clusters in variation diagrams of Cu and δ65Cu with mass fractions of incompatible elements. This clustering suggests limited fractionation of Cu isotopes, hence an evolution of most MB suites as individual systems, and points towards heterogeneous mantle sources. Thus, the δ65Cu differences between low-Ti and high-Ti MB likely reflect Cu isotope partitioning between silicates melt and alloys evolving from S-poor to S-rich at different stages of LMO crystallization. On the other hand, the more limited δ65Cu variation within MB suites relate to processes during parental melt evolution after melting of lunar mantle sources. Consequently, δ65Cu of lunar MB dominantly record processes prior and/or during solidification of mantle source and their bearing on volatility related processes is limited.
References: 1. LPSE-Team° (1969). Sci 165:1211-1227. 2. G. F. Herzog et al.° (2009). GCA 73:5884-5904. 3. J. M. Day, F. Moynier° (2014). Philos Trans A Math Phys Eng Sci 372:20130259. 4. E. A. Pringle, F. Moynier° (2017). EPSL 473:62-70. 5. K. Wang, S. B. Jacobsen° (2016). GCA 178:223-232. 6. C. Kato, F. Moynier° (2017). Sci Adv 3:e1700571. 7. K. Pahlevan, D. J. Stevenson° (2007). EPSL 262:438-449. 8. C. Kato et al.° (2015). Nat Com 6:7617. 9. N. Dauphas et al.° (2022). Planet Sci Jour 3. 10. P. Gleißner et al.° (2022). EPSL 593. 11. H. M. Williams, C. Archer° (2011). GCA 75:3166-3178.
How to cite: Florin, G., Gleißner, P., and Becker, H.: Mantle Source Heterogeneities in the Moon Revealed by Copper Isotopic Compositions of Mare Basalts, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-929, https://doi.org/10.5194/epsc2024-929, 2024.
Long-wavelength topography variations are observed on telluric planets. They are often associated with long-wavelength crustal thickness variations and define different geological provinces. Mars and the Moon both display a hemispherical dichotomy in topography. The Southern Hemisphere of Mars and the far side of the Moon constitute the highlands of the planets, while the opposite hemispheres concentrate topographic lows: the Martian plains and the Lunar mare. Venus also shows highlands surrounded by volcanic plains, but no hemispheric dichotomy. On Earth, the present-day difference in topography between continents and oceans is not hemispheric either and is, in this case, the result of plate tectonics. Mercury seems to be an exception, as it does not present long-wavelength variations in topography. However, a thick crustal province associated with a denser crust has been proposed.
On one-plate planets, a positive feedback mechanism exists between crustal thickness and melt extraction. Thicker crusts contain more radioelements, which leads to hotter temperature profiles and hence thinner lid thicknesses. Because of the pressure-dependence of the solidus, thinner lids are associated to more partial melt in the mantle below the lid and, hence, larger crustal extraction rates where the crust is thicker. This mechanism enables the spontaneous formation of thick crustal regions (Bonnet Gibet et al. 2022). A linear stability analysis on a simplified setup that does not account for planetary cooling shows that long-wavelength variations in crustal thickness, the hemispherical one (degree l = 1) in particular, are favoured by this mechanism. Shorter wavelengths are more efficiently attenuated by lateral diffusion of heat within the conductive lid (Figure 1, left panel). Additionally, as the planetary radius gets smaller, the growth rate becomes more peaked towards the degree l=1 mode (Figure 1, left panel). A hemispherical difference, or dichotomy, in crustal thickness is thus even more likely to grow on a smaller planet. The growth of a perturbation in crustal thickness necessitates, however, the presence of partial melt and is thus dampened by planetary cooling and lid thickening. Therefore, if the timescale for crustal extraction is too short, i.e. shorter than the characteristic timescale for the growth of this perturbation, long wavelength variations in crustal thickness cannot grow through this mechanism (Figure 1, right panel).
Figure 1 : Left panel, growth rate of a perturbation normalised by the growth rate l=1 (λ(l)/λ(1)) as a function of spherical harmonic degree (l) for different planet sizes. Right panel, characteristic timescale for the growth of a perturbation (1/λ(l)) as a function of planetary radius.
We use a parametrised, asymmetric, thermal evolution model accounting for crustal extraction. We assume a thermally well-mixed convective mantle topped by a stagnant lid divided into two different hemispheres that evolve independently. This model allows us to calculate the temporal evolution of the lid thickness, the mantle melt fraction and the crustal thickness for each hemisphere. Assuming a very small initial thermal anomaly between the two hemispheres, we monitor the thermal evolution of the planet to see whether this small difference grows into a crustal thickness dichotomy or decays. We apply our model to Mercury, Mars, and Venus, with an initial thermal state corresponding to the end of the crystallisation of a magma ocean.
Our results show that this positive feedback mechanism can explain by itself the formation of the Martian dichotomy for a large range of parameters (Bonnet Gibet et al. 2022). For Mars, the crust extraction time is indeed larger than 1 Gyr and hence larger than the characteristic time for the growth of the initial thermal perturbation. For Venus, as the planet's secular cooling is very small, crustal extraction can proceed throughout the planet's evolution, but at a lower mantle melt fraction because of the larger internal pressure gradient. Variations in the crustal thickness can thus develop on Venus during an episode of stagnant lid convection with this mechanism. It is, however, not clear whether the degree l=1 mode could be selected, given the very flat growth rate curve at a small spherical harmonic degree (Figure 1). On the contrary, for Mercury, which has a thin silicate shell (~400 km) and a high Urey ratio, cooling is too fast for this mechanism to produce a significant dichotomy in crustal thickness. Mars seems therefore to present the optimal size for the growth of a hemispheric crustal dichotomy.
How to cite: Bonnet Gibet, V. and michaut, C.: The origin of long-wavelength variations in crustal thickness on telluric planet, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-909, https://doi.org/10.5194/epsc2024-909, 2024.
The lunar anorthosite highlands, composed of 90 wt% anorthite (Ca-rich plagioclase), represent the Moon’s primary crust [1], which formed during the solidification of a magma ocean following the Moon-forming giant impact between the proto-Earth and Theia. The composition of this anorthite-enriched crust is unique among the terrestrial bodies within our Solar System and can be easily reproduced by crustal formation due to the flotation of buoyant anorthite crystals during magma ocean solidification [2] (Fig. 1a). However, this canonical model often struggles to reproduce the long, > 200 Myr solidification timescale required by the ages of the anorthosite suite [2], as well as the diverse magmatic histories of the lunar anorthosites [3].
If, instead, the anorthite crystals remain entrained in magma ocean due to vigorous convection, the magma ocean behaves as a liquid-crystal slurry with an immobile stagnant lid at the surface (Fig. 1b), which reduces the cooling rate of the magma ocean and results in long solidification durations [3]. In this regime, crustal formation occurs via extraction of melt from the magma ocean through the stagnant lid. In this study, we investigate whether sufficient magmatic differentiation can occur within the stagnant lid to produce an anorthite-rich crust. Our model consists of a simplified two-component anorthite-olivine melt system, which becomes progressively enriched in anorthite as the melt solidifies. We calculate the thermochemical evolution of this melt as it rises through a growing stagnant lid, cools, and solidifies for a range of Peclet numbers (the ratio of the rate of advection to diffusion), and lid thicknesses.
We find that for low Peclet numbers (<1), little differentiation and melt accumulation occurs within the lid, regardless of lid thickness, and no anorthosite crust formation is likely possible. On the other hand, for the high Peclet numbers relevant to the lunar magma ocean (>30), thick, melt- and anorthite-rich (>30 vol%) sills can form within the lid and migrate slowly upwards. However, the composition of these sills is still more olivine-rich than that observed in the lunar highlands. Therefore, further differentiation is likely required within the sills themselves. This could occur by the flotation of anorthite crystals as this is a process that is more likely to occur in these emplaced magmatic bodies than at the scale of the magma ocean. Equally none of these sills are predicted to reach the cold, solid surface of the lid. However, we find that the overpressure within the sills is sufficient to drive the eruption of this melt to the surface, leading to a crustal formation by a combination of extrusive and intrusive magmatism.
[1] Wood et al., 1970, in Proceedings of the Apollo 11 Lunar Science Conference (pp. 1965-1988)
[2] Elkins-Tanton, L.T., Burgess, S. and Yin, Q.Z., 2011. The lunar magma ocean: Reconciling the solidification process with lunar petrology and geochronology. Earth and Planetary Science Letters, 304(3-4), pp.326-336.
[3] Ashwal, L.D. and Bybee, G.M., 2017. Crustal evolution and the temporality of anorthosites. Earth-Science Reviews, 173, pp.307-330.
[4] Michaut, C. and Neufeld, J.A., 2022. Formation of the lunar primary crust from a long‐lived slushy magma ocean. Geophysical Research Letters, 49(2), p.e2021GL095408.
Fig. 1. Sketches of a) the classical anorthite flotation crust model and b) the slushy magma ocean model where the crystal fraction remains entrained by convection and a crust forms by melt extraction in a stagnant lid.
How to cite: Dodds, K., Michaut, C., and Neufeld, J.: Lunar crustal formation by melt migration and differentiation within a stagnant lid., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-870, https://doi.org/10.5194/epsc2024-870, 2024.
The origin of the most primitive, picritic lunar basalts, sampled as pyroclastic glass beads in the lunar soils [1,2], remains poorly constrained. Especially the petrogenesis of high-Ti glasses (TiO2 > 6 wt%) seems enigmatic. Three hypotheses have been proposed for the origin of these exotic samples: I) Ascent of a primary, undifferentiated melt from the lunar interior and assimilation of clinopyroxene and ilmenite in the upper section of the not overturned lunar mantle [3]. II) Melting of a hybridized lunar mantle after lunar mantel overturn [4,5]. III) Reaction of sunken low-degree high-density melts of the hybrid magma ocean source with high Mg-cumulates in the deep interior of the lunar mantle and subsequent ascent [5,6,7]. This study re-investigates hypothesis II with the aim to assess whether a one stage melting process of a heterogeneous lunar mantle can cause the compositional variabilities of lunar (high-Ti) picritic glasses. Specifically, the effect of modal mineralogy of different cumulate layers in the hybrid lunar mantle is investigated.
The overturn of the lunar mantle stratification due to Rayleigh-Taylor instabilities will have caused the so-called “Ilmenite-bearing cumulate (IBC)” to sink into the underlying harzburgite lunar mantle [7]. Therefore, an IB cumulate and a harzburgite cumulate appear to be the major components of a hybrid lunar mantle [7]. In this study, the first batch of starting material compositions was mixed similar to [5] using a fixed composition the harzburgite mantle. An IBC was designed by mixing ilmenite, clinopyroxene, and small amounts of plagioclase. These minerals are the basic components of the bulk lunar mantle after cumulate overturn [2]. We investigated how the ilm/cpx ratio within an IBC will affect the melt compositions and melting conditions. A such, we assumed that modal proportions of crystallization were not preserved in the cumulate [e.g., 5,9]. In a second batch of experiments, we slightly adjusted olivine and orthopyroxene ratios in the harzburgite layer. A third component of a hybrid lunar mantle in some of our starting material composition was an urKREEP component [10], which has been proposed to participate in the overturn and melting process [7,11].
To investigate the composition and modal amounts of partial melts from several different starting materials, we conducted high-pressure and high-temperature experiments in an endloaded Piston cylinder apparatus at the University of Münster. Most runs were conducted at a pressure of 1.5 GPa, which corresponds to the lower end of the depth range suggested for the source depth of high-Ti lunar picritic basalts [6]. Run temperatures were varied between 1300 and 1450 °C to investigate the effect of changing melting degree on melt compositions [3]. In order to control fO2 and to minimize Fe-loss in the runs, we used graphite-lined Pt capsules [3,4]. The characterization of experimental runs was conducted by the means of scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). Mineral and melt proportions were determined via mass balance calculations using the major element chemistry of the phases present in each experiment.
All experiments contain glass and forsteritic olivine. Some experiments contain olivine and orthopyroxene. The degree of partial melting (Fmelt) is 0.18–0.75. The experiments that contain both olivine and orthopyroxene were run at lower temperatures (< 1400 °C) and low degrees of partial melting (Fmelt < 0.5).
Figure 1: SiO2 vs. TiO2 of lunar Apollo picritic glasses (yellow, orange, red, black – colored accordingly) from [8,9] and our experimental melts (squares and circles). Indivudual symbols correspond to different starting material compositions.Triangles correspond to melting experiments of a hybrid lunar mantle with modal ilm/cpx from fractional crystallization experiments [5]
Comparing the composition of experimental melts and natural lunar picritic glasses (Fig. 1), it can be stated that the melting of a heterogeneous lunar mantle produced by the overturn of lunar stratification after the solidification of the lunar magma ocean can generate melts in the range similar high-Ti picritic melts. Experimental temperatures and pressures agree with the temperatures and depth of origin predicted by previous experimental studies [3,5]. A partial melting process of a cumulate-bearing mantle, as modeled by our experiments, is shown to be a viable and simple alternative to the currently accepted complex melting model [5,6]. In our experimental setup, a ratio of ilm/cpx of 1/1 or 4/1 in the IBC layer reproduces high-Ti compositions, similar to the picritic lunar basalts. Good matches are achieved in runs with lower temperatures, which correspond to the comparably lower degree of melting.
The most suitable ol/opx is 3/2. We further find that, following the constraints of [12], some plagioclase has to be entrained in the IBC layer, in order to reproduce Al2O3/CaO in the cumulate mantle melting experiments. Additionally, the presence of urKREEP in the cumulates strongly influences Al2O3/CaO, driving it too high in melts originating from cumulates containing that component.
In the light of our experiments, it is possible to shed some new light on the origin of exotic lunar basalt samples, such as the picritic, high-Ti lunar basalts. We explored the feasibility of a simple melting process of a hybrid lunar mantle after overturn.
[1] Delano (1986) J Geophys Res-Solid 91, B4 201-213 [2] Shearer C. K. and Papike J. J. (1993) Geochim Cosmochim Ac, 57, 19, 4785-4812 [3] Wagner T. P. and Grove T. L. (1997) Geochim Cosmochim Ac, 61, 6, 1315–1327. [4] Krawczynski M. J. and Grove T. L. (2012) Geochim Cosmochim Ac, 79, 1-19. [5] Singletary G. H. and Grove T. L. (2008) Earth Planet Sc Lett, 208, 182–189. [6] Elkins-Tanton L. T., van Orman J. A. et al. (2002) Earth Planet Sc Lett, 196, 239-249. [7] Hess P. C. and Parmentier E. M. (1995) Earth Planet Sc Lett 134, 501-514 [8] Elkins L. T., Fernandes V. A. et al. (2000) Geochim Cosmochim Ac, 64.13, 2339–2350. [9] Brown, S. M., Grove, T. L. (2015). Geochim Cosmochim Ac, 171, 201-215. [10] Warren, P. H., Wasson, J. T. (1979). Rev Geophys, 17(1), 73-88. [11] Elardo, S. M., Laneuville, M. et. al (2020). Nat Geosci, 13(5), 339-343. [12] Charlier, B., Grove, T. L., et al. (2018). Geochim Cosmochim Ac, 234, 50-69.
How to cite: Haupt, C., Renggli, C. J., Rohrbach, A., Berndt, J., and Klemme, S.: Resolving the origin of lunar high-Ti basalts by petrologic experiments, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-763, https://doi.org/10.5194/epsc2024-763, 2024.
Introduction: Because of early recurring impacts and later impact gardening, lunar crust surface turned into a layer of fragmented, variably shocked and occasionally melted impact breccias [Heiken et al., 199; White et al., 2020].
One of the most severely affected group of rocks found on the lunar surface is the Mg-suite group. These are Mg-rich, primitive, plutonic to hypabyssal coarse-grained rocks which texture and bulk composition reflect magmatic accumulation of mineral phases [Heiken et al., 1991; White et al., 2020; Shearer et al., 2015; Černok et al., 2020].
Whether the Mg-suite rocks formed by partial melting of the lunar mantle or they are impact-related is still debated [Taylor et al., 1993]. In the latter case, their origin could be related to the melting of early lunar crust and mantle caused by frequent hypervelocity impacts [Jolliff et al., 2006].
Some of their minerals, like Fe-Ni metals and sulfides, reflect mixing and melting of impactor(s) and target rocks and the proportion of impactor vs. target contributions [Tang et al., 2023; Day et al., 2020]. For example, a high Ni/Co ratio in Fe-Ni metal grains can be the consequence of the addition of either iron or chondritic impactors. Furthermore, those same minerals are extremely sensitive to processes that happened in Moon’s interior, such as fractional crystallization [Day et al., 2020]. For example, Ni/Co ratio decreases when mineral phases like pyroxene and plagioclase crystallize together because of the different compatibility of Ni and Co in both solid and melt.
For these reasons, those minerals cannot only be considered important means to investigate lunar rocks origin, but also lunar evolution processes [Day et al., 2020]. In this study we concentrate on defining the mineral chemistry of Fe-Ni metal and sulfide grains in a set of variably shocked breccias from three different Apollo missions (15, 16 and 17).
Figure 1. Fe-Ni metal grain in thin section 78235,38, BSE image
Samples: Three different shocked breccia samples were selected from Apollo 15, 16 and 17 collections: shocked norite 78235 (thin sections 78235,38 and 78235,51), mainly consisting of glass veins and cumulus and partially fractured orthopyroxene and plagioclase, with much of the plagioclase converted into maskelynite [Meyer, 2010]; dimict breccia with shocked norite 15455 (thin sections 15455,27 and 15455,28), primarily made of fragmented orthopyroxene and plagioclase in a KREEP-rich, fine-grained igneous-textured groundmass containing plagioclase, olivine and pink spinel clasts [Meyer, 2010]; feldspathic polymict breccia 67915, showing two-main polymict lithologies, one white and one grey, containing heterogeneous lithic clasts cemented in shock-melted glass (thin sections 67915,76 and 67915,84) [Meyer, 2010].
Methods: Non-destructive chemical analysis were performed at the Institute of Geological Sciences at the Freie Universität Berlin (Germany) using a JEOL JXA 8200 Superprobe on minerals selected with the help of QEMSCAN maps and BSE images. In both metals and sulfides we analyzed the concentration of siderophile (Fe, Ni, Co, Mn), chalcophile (S, Zn, Cu) and some lithophile elements (such as Ca, Mg, Si, Cr, P, used to evaluate possible interference by surrounding silicates). For all the elements detection limits lie between 100 and 200 ppm, while the beam size was usually 1 μm.
Figure 2. Sulfide and Fe-Ni metal grains in thin section 78235,51, BSE image
Initial results: In total, more than 150 grains were analyzed in four different thin sections, but most of them were very small (<1 µm), thus not suitable for chemical analyses. In sample 78235 three metal grains found in glass veins from 78235,51 show higher Ni/Co (from ~10.5 to ~20) and lower Fe/Ni (from ~3.6 to ~10.2) ratios. Remaining metal grains found in glass veins or in cumulus plagioclase (maskelynite) and/or pyroxene from the host norite show lower Ni/Co (from ~0.8 to ~0.9) and higher Fe/Ni (from ~35 to ~45) ratios. Similarly, sulfide grains generally plot within a restricted Ni/Co (from ~0.3 to ~0.5) and Fe/Ni (from ~1300 to ~2100) range of values, but grains from maskelynite tend to cover a wider range (Ni/Co from ~0.1 to ~1, Fe/Ni from ~52.5 to >6700). Different is in 78235,38, where all metal grains plot within a small range of values (Ni/Co from ~0.7 to ~0.8, Fe/Ni from ~41.8 to ~48.7), while sulfide grains generally show a higher Ni/Co (from ~1.4 to ~3.1) and Fe/Ni (from ~1600 to >6800).
In sample 15455 most of the selected grains were found in the impact melt, but are usually <1 μm. The larger, albeit rare, grains (larger than 7 x 7 μm) were found in plagioclase and orthopyroxene clasts. In sample 15455, as in 78235,51, metal grains in the groundmass show variable Ni/Co (from ~0.8 to ~29.3) and Fe/Ni (from ~6.4 to ~80.2) compared to the ones found in plagioclase (Ni/Co from ~0.8 to ~2.1, Fe/Ni from ~5.1 to ~15) and orthopyroxene (Ni/Co from ~0.6 to ~0.8, Fe/Ni from ~18.1 to ~57.7). Sulfide grains show slightly higher Ni/Co (from ~0.6 to ~15 for pyroxenes, from ~2.3 to 5 for plagioclase) and lower Fe/Ni (from ~480 to >4700 for pyroxenes, from ~9.3 to >4700 for plagioclase) compared to sulfides in 78235.
In sample 67915 chemical analysis are yet to be performed.
Preliminary conclusions: The calculated Ni/Co and Fe/Ni from the investigated Fe-Ni metals in differently shocked Apollo Mg-suite samples indicate that at least some are of possible impact-related origin. Fe-Ni metal grains within glass veins in sample 78235 and matrix in sample 15455 usually show Ni/Co ~20, compatible with possible iron meteorite contamination, as shown by Day et al (2020) and McCallum and Mathez (1975). FeNi metal grains belonging to other mineral assemblages have lower Ni/Co, usually ≤1, which is more compatible to an endogenous origin [Day et al., 2020]. This is also evident in sulfides, were a Ni/Co ratio that varies between ~0.3 and ~5 suggest a more endogenous origin. Fe/Ni values in Fe-Fe-Ni metals are mostly compatible with kamacite metal (>92% Fe, <7% Ni) [Day et al., 2020].
How to cite: Sist, E., Černok, A., Beinlich, A., Dürr, T., and Becker, H.: Investigating the origin of Fe-Ni metal and sulfides in shocked Apollo Mg-suite rocks , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1083, https://doi.org/10.5194/epsc2024-1083, 2024.